This being my first introduction to modal logic, I will not be the most accurate reviewer, I'm sure. Additionally, I can't compare this text to others on the subject.
Overall, the book was pretty accessible. But I felt that Konyndyk could have been more lucid at times. I normally do not struggle to understand texts, but I did at times with this book. Perhaps that's just the nature of modal logic for me.
My biggest beef, though, is that there are a number of errors throughout in the proofs/exercises. They are probably typos rather than actual proof errors. And perhaps the newer edition has fixed these errors (unfortunately, both editions have the same ISBN, so buying used from a place like Amazon does not guarantee which edition you'll receive).
Perhaps there are no better introductory texts for modal logic, but it certainly seems that this text can be improved upon.